1. Field of the Invention
The present invention relates to a fluorescent lamp, and more particularly, to a fluorescent lamp having improved auxiliary anodes which prevents a flickering phenomenon and enables the lamp to withstand long use.
2. Description of the Related Art
In recent years, a compact fluorescent lamp employing a smaller-diameter envelope than hitherto has been put to practical use. Although this type of fluorescent lamp is useful in practice, it has the drawback that the mercury sealed therein is hard to ionize since the number of electrons existing in the vicinity of the anode is small. In addition, since the volume of the discharge space is small, the impurity gas concentration is high, accordingly. If the impurity gas concentration is high, the anode is likely to cease oscillation, causing a flickering phenomenon.
Published Unexamined Japanese Patent Applications (PUJPA) No. 60-14740 and No. 62-219435 disclose an example of a fluorescent lamp wherein the flickering at the electrodes is suppressed. In the fluorescent lamp disclosed in the former Application, an auxiliary anode is located in the vicinity of a filament coil and is projected in the discharge direction of the lamp such that its distal end is located away from the central axis of the filament coil by 5 mm or more. In the fluorescent lamp disclosed in the latter Application, an auxiliary electrode is projected in the discharge direction such that its distal end is located away from the central axis of the filament coil by 4 mm and such that it is located between two internal conductive wires. Due to the use of such auxiliary electrodes, flickering is remarkably suppressed in the fluorescent lamps disclosed in the two Applications.
In the fluorescent lamps disclosed in the two Applications, however, electrons are likely to flow toward the auxiliary electrodes (i.e., anodes), not toward the filament, with the result that the temperature at the cathode spots decreases. Thus, the discharge becomes unstable, and sputtering occurs markedly at the emitter of the filament and at the metal of the electrode. Accordingly, the life of the fluorescent lamps is adversely affected, due to the consumption of the electrodes and the blackening of the tube.
If the conventional fluorescent lamp is high-frequency lit by use of an inverter circuit, a very large amount of anode current flows to the auxiliary electrode (anode). Since, therefore, only a small amount of current flows to the filament, the temperature of the filament does not become high. In such a case, the discharge becomes unstable, thus causing half-wave discharge wherein discharge occurs only at one of the electrodes. Accordingly, a so-called cataphoresis phenomenon occurs.